Apache Request Size

I got an issue relating to a costumers website.
I've a couple of thumbnails on this website and they're not going to be loaded (403 forbidden) when you open the page: http://www.retouche.codag.net/portfolio

After a couple of page refreshes the thumbnails will be displayed.

So I guess there's a problem of request size which is too big.

Could this be the issue or do you guys have an other idea?
How can I increase the max. request output? Is it max request lenght?